A high grip performance is demanded for tires as a basic property. It is generally known, as a method for improving a grip performance of tires, to blend in a rubber composition for a tire a rubber having a high glass transition temperature (Tg) (for example, one having Tg of −25° C. or more) or carbon black having a large surface area. In order to make a glass transition temperature high, for example, there is a method of increasing a styrene content in a polymer. However, when a copolymerization ratio of an aromatic vinyl compound such as styrene becomes as high as 45% by mass or more or when a molecular weight (Mw) of a polymer becomes as high as more than 500000, there is a problem, for example, that a styrene chain distribution in a polymer increases, and a temperature dependence curve (a curve obtained by plotting tan δ values at each temperature when the temperature is changed) of a viscoelasticity tan δ of a rubber composition for a tire prepared using such a polymer shows multiple peaks.
For vehicles, especially general cars such as passenger cars, low fuel consumption, namely improvement in rolling resistance is demanded from environmental point of view in addition to a grip performance, in particular a wet grip performance as a basic performance. A viscoelasticity tan δ of a rubber composition is an index for such wet grip performance and low fuel consumption. Namely, a tan δ at 0° C. is an index for wet grip performance, and the higher the tan δ is, the better a braking efficiency is. A tan δ at 60° C. is an index for rolling resistance, and the lower the tan δ is, the better the fuel consumption is.
In addition, for vehicles for racing, a high grip performance is demanded as a basic performance. A viscoelasticity tan δ of a rubber composition is an index for such grip performance. Namely, a tan δ in a temperature range of from 20° to 100° C., particularly from 30° to 45° C., is an index for grip performance within such a temperature range, and the higher the tan δ is, the better a braking efficiency is.
Therefore, if a temperature dependence curve of tan δ can be controlled so that its shape becomes as uniform as possible without multiple peaks, it is possible to provide a rubber composition for a tire exhibiting intended desired characteristics (for example, a rubber composition for a tire having improved grip performance within a specific temperature range, or the like). Therefore, in the development of a polymer for a tire rubber, control of a temperature dependence curve of tan δ is an important issue to be addressed.